1. Field of the Invention
This invention relates to a vesicle dispersion utilizing a readily available nonionic surfactant, which is capable of maintaining a hydrophilic or hydrophobic effective component in an isolated state from an aqueous dispersion medium.
2. Description of the Prior Art
It is known in the art that an amphiphatic substance can form vesicles in water. For example, vesicles such as liposomes based on phosphatides or phospholipids and ufasomes based on unsaturated fatty acids are present in natural substances. Attempts have been made to use these natural vesicles in the fields of cosmetics and pharmaceuticals since these types of vesicles are stable dispersions and are safe to use. However, these natural vesicles are not suitable for use in large volume consumption because of their relatively high cost.
Recently, vesicles or niosomes utilizing readily available nonionic surfactants have been found. For example, it has been reported in Japanese Unexamined Patent Publication (Kokai) No. 52-6375 that vesicle dispersions are formed from nonionic surfactants having the general formula: ##STR1## wherein R represents an aliphatic hydrocarbon group having 12 to 30 carbon atoms and n represents an integer of 1 to 6. It has also been reported in J. Colloid Interface Sci., Vol. 82, No. 2, 401-417 (1981) that vesicles are formed from ethylene oxide addition products of glycerin dialkyl ethers or ethylene oxide addition products of myristic acid stearyl amides. On the other hand, it has been reported in Nippon Kagaku Kaishi, 1981 (11), p. 1691-1696 that polyoxyethylene hardened castor oil ethers and polyoxyethylene sorbitol tetraoleate form concentric lamella type liquid crystals.
Vesicles can be considered to be special forms of concentric lamella type liquid crystals. Vesicles are different from the concentric lamella type liquid crystals since water or an aqueous solution can be contained in substantially hydrophillic cavities formed in the inside of bimolecular or multiple layer membranes (or films) formed from a surfactant. Accordingly, it is necessary that the surfactant molecules be oriented so as to form lamella bimolecular membranes having a curvature capable of readily forming vesicles. However, although the above-mentioned polyoxyethylene hardened castor oil ethers form concentric lamella type liquid crystals, the polyoxyethylene hardened castor oil ethers cannot form uniform state concentric lamella type liquid crystals and clear vesicle structures due to the distribution of the ethylene oxide addition mole number, since the polyoxyethylene hardened castor oil ethers are mixtures of the plural compounds having different ethylene oxide addition mole number.